Soot sensors, also known as particulate matter (PM) sensors, may be used in vehicles having diesel engines. A particulate matter sensor may be located upstream from a diesel particulate filter (DPF), where the sensor is exposed to exhaust flow from the engine having soot particles entrained in the exhaust gas. Alternatively, a particulate matter sensor may be located in a gas stream downstream from a diesel particulate filter (DPF), where the sensor is used to monitor the proper operation of the particulate filter.
A known method of sensing soot uses a particulate matter sensor having two electrodes that are spaced from one another. In the absence of soot, there is very low electrical conductivity (high electrical resistance) between the electrodes. As soot accumulates on the surface of the sensor, soot particles act to bridge the gap between the electrodes. Because the soot particles are electrically conductive the conductivity between the electrodes increases, and this change in conductivity can be related to the amount of soot in the gas stream. Sensors that operate according to this principle are disclosed in U.S. patent application Ser. No. 11/749,262 published as US Patent Application Publication 2008/0283398, U.S. patent application Ser. No. 11/750,883 published as US Patent Application Publication 2008/0282769, and U.S. patent application Ser. No. 11/998,238 published as US Patent Application Publication 2009/0139081, the contents of all of which are hereby incorporated by reference in their entirety.
Government regulations require that the vehicle system must be able to detect when a DPF has failed. The particulate matter sensor algorithm determines this by measuring the rate of soot that passes the sensing element. The particulate matter sensing environment may contain sources of significant noise. These noise sources are primarily voltage-based noise (i.e. EMI or conducted ground noise) and soot based noise (i.e. large particulate matter particles or agglomerate particulate matter loss). The noise on the particulate matter sensor signal can interfere with the ability to detect when a DPF has failed.
Accordingly, the inventors herein have recognized a need for an improved sensing system having a particulate matter sensor that reduces and/or eliminates the foregoing deficiencies.